Servo-assisted mechanical transmissions, which are structurally similar to a manual mechanical transmission of the traditional type except for the fact that the clutch pedal and the gear selection lever operated by the driver are replaced by corresponding electric or hydraulic servo-controls, are increasingly widespread.
Generally, the clutch servo-control is of the hydraulic type and comprises a single hydraulic actuator for displacing the clutch from the closing position to the opening position and vice versa. Generally, the gearbox servo-control is also of the hydraulic type and acts on a gearbox control shaft to impress on the control shaft itself both an axial displacement, i.e. along a central axis, and a rotation about the central axis; the two movements of the control shaft are necessary to engage and disengage each gear and to select the range of gears to be engaged. Consequently, the gearbox servo-control comprises a first hydraulic actuator mechanically coupled to the control shaft to axially displace the control shaft and a second hydraulic actuator mechanically coupled to the control shaft to rotate the control shaft.
Each hydraulic actuator comprises at least one actuation chamber delimited on one side by a mobile piston, which slides along the actuation chamber itself and is mechanically connected to the control shaft. Furthermore, for each actuation chamber, the hydraulic actuator comprises at least one three-way solenoid valve which drives the hydraulic actuator and which is adapted either to connect the actuation chamber to a hydraulic accumulator containing pressurized oil, or to connect the actuation chamber to a discharge tank containing oil at atmospheric pressure, or to maintain the actuation chamber isolated.
Normally, the hydraulic actuator which controls the gear engagement/disengagement is driven by using a pressure-controlled proportioning solenoid valve, which allows to very accurately control the force which is developed by the hydraulic actuator and applied to the control shaft (the oil pressure inside the actuation chamber is directly proportional to the force developed by the hydraulic actuator); indeed, in the engagement/disengagement of the gears it is essential for the hydraulic actuator to develop a force suitable for overcoming the mechanical resistances, otherwise the gears cannot be engaged/disengaged and the control shaft does not move. However, with a pressure-controlled proportioning solenoid valve it is not possible to accurately control the instantaneous position of the control shaft actuated by the hydraulic actuator and therefore positioning errors of the control shaft, which cause undesired noise, may easily occur. In order to accurately control the instantaneous position of the control shaft actuated by the hydraulic actuator, a flow-controlled proportioning valve should be used (the amount of oil inside the actuation chamber is directly proportional to the position of the hydraulic actuator); however, by using a flow-controlled proportioning solenoid valve it is not possible to accurately control the force which is developed by the hydraulic actuator and thus it is not possible to guarantee that the force which is developed by the hydraulic actuator is sufficient to engage/disengage the gears.